1. Field of the Invention
The present invention generally relates to pH indicators and more specifically to a novel type of pH indicator sticks coated by substances having pH-depending solubility properties, for the detection of pH transitions.
2. Discussion of the Related Art
There are many cases where a change in the pH level below or above a certain value reflects a crucial change of a condition or a crucial stage during a process. For example, pH of bodily fluids and secretions are used for diagnosing various biological conditions. A deviation from the normal value may indicate abnormal condition. For example, studies have shown that pH of vaginal fluid which is equal to or greater than 4.6 may be a symptom of bacterial vaginosis or candida. Another example is food products where the drop of the pH level may indicate spoilage of food. Although it is now mandatory in most countries to indicate an expiration date on food products, the indicated expiration date is estimation only and does not necessarily reflect the real status of the particular food product. Many more other examples are known in various industrial applications.
Accurate measurements of pH are conducted potentiometrically by using a pH-meter with pH-selective electrode. However, such measurements are not always feasible or worthwhile. Moreover, many times it is sufficient to only know whether the pH falls below and/or elevates above a specific threshold value while it is of less importance to know the exact pH value before the transition occurs. For this reason it is a common practice to use pH indicators which have different color at different pH values. pH indicators having pH dependent color are well known for many years. They consist of compounds, usually weak bases or acids that, due to differences in electronic configuration, have a different visible color in their neutral and charged forms. Small amounts of such pH indicator compounds can be added directly to test samples or alternatively, the compounds are impregnated onto a substrate, typically paper strips, to form pH indicator strips. However, the reading and interpretation of such pH indicators is subjective. Interpretation is particularly ambiguous at the pH range where the indicator turns its color (pKa) i.e., where both forms of the compound are present. Moreover, indicator strips tend to lose their pH indicative color upon drying, therefore reading has to be made immediately which is not always possible.
The present invention provides a new type of pH indicators which is based on pH dependent solubility properties. The indicators of the invention give a clear visible signal (a color signal) when a predetermined pH change occurs. The invention provides a full spectrum of indicators which can be designed for a specific pH transition and specific use. By selecting the appropriate substances, the novel indicators may be adjusted to accurately detecting any desirable pH change. For example, suitable substances may be selected to provide a pH indicator that changes color only in the range 4<pH<6 but maintains its default color in all other pH values. Another type of indicators may be prepared that give a signal when the pH changes above or below one or more predetermined thresholds. For example, by selecting the appropriate substances, an indicator may be designed to react only when pH>10 or pH<3 but remains unchanged when 3<pH<10. Moreover the novel indicators may be tailored to indicate a particular pH change in an expected non-monotonic pH behavior. For example if during a process the pH is known to first elevate and then to fall again so as to pass through the same pH value twice but it is only the second time that should be indicated.
The pH indicators of the invention may be stand alone indicators configured as sticks or strips to be immersed into a test sample, or may be incorporated into a variety of articles including, but not limited to, food products and disposable absorbent articles such as hygienic pads and panty shields.